Fluoride ion selective electrode

ABSTRACT

A fluoride monitoring electrode comprises a single crystal of a lanthanum series fluoride doped with alkaline earth ions. The sample pre-treatment solution used in conjunction with the electrode includes a buffer that maintains a pH of 5 to 8 and a complexing agent that complexes iron and aluminum.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the measurement of fluoride ions in an aqueousmedium. More specifically, it relates to a fluoride ion-selectivesensing electrode made of a single crystal of lanthanide fluoride dopedwith alkaline earth ions. The invention also relates to a samplepretreatment solution containing buffering and complexing agentssometimes referred to as TISAB (total ionic strength adjustment buffer)incorporated in the sample solution monitored by means of the electrode.

2. Background Information

For a number of years, ion-selective electrodes have been used tomeasure the concentration of fluoride ions without substantialinterference from other ions present in the same solution. The voltagedeveloped across an electrode exposed on one side to a sample solutionof fluoride ions and, on the other side, a standard solution, iscompared with the voltage developed by an electrode exposed to areference solution, the voltage difference corresponding with thefluoride ion concentration in the sample solution. Since at least 1966,it has been known to use a fluoride ion-selective electrode employingthe crystalline trifluoride of a metal of the lanthanide series (Frantand Ross, Science, vol. 154, 1553-1555 (1966); Frant, U.S. Pat. No.3,431,182 (1969)). As set forth in the Frant patent, the sensingelectrode is termed a “membrane,” consistent with its usage inpotentiometric electrode technology. It embraces a non-porous sheet-likestructure, generally regardless of flexibility or curvature, whichprovides a pair of limiting surfaces between which charge transfer iseffected. The membranes disclosed by Frant and Ross are single crystalsof pure lanthanum triflouride and also single crystals of lanthanumtrifluoride doped with europium trifluoride. The latter combinationexhibits low membrane resistance and is the most widely usedsingle-crystal lanthanum membrane for fluoride-sensing electrodes.

The literature also includes descriptions of non-crystalline lanthanidemembranes. For example, Kobos et al., U.S. Pat. No. 4,931,172 (1990),describe sintered membranes of the form (MF₂)_(1-x)(LnF₃)_(x) where M isan alkaline earth metal, such as calcium, strontium or barium, and Ln isa lanthanide series metal, such as lanthanum, cerium, praseodymium,europium, etc.

The electrodes also usually work with a sample pretreatment solutionthat maintains the pH at around pH 5.4, thereby limiting the effect ofpH changes and OH⁻ interference, which occurs at high pH values, and HFwhich occurs at or below pH 5, reducing the fluoride ion activity in thesolution. A widely used buffer has been acetate with pH range from 5 to5.5. Although acetate is widely used as a sample pretreatment solutionfor fluoride measurement due to its excellent buffer nature for the pHrange of 5 to 5.5, it increases the response time of the electrode,decreases the sensitivity, and deteriorates the detection limits of theanalysis (Anfalt, T. and Jagner, D., Anal. Chim. Acta., 47, 483-494(1969); Anfalt, T. and Jagner, D., Anal. Chim. Acta., 50, 23-30 (1970)).For this reason, the literature describes the use of amorpholinoethanesulfonic acid-based buffer that improves the detectionlimit of fluoride ion-selective electrodes in the pH range of 5 to 5.5.However, there is no mention of using similar biological organic acidbuffers beyond the pH range of 5 to 5.5 (Fouskaki M., Sotiropoulou S.,Kochi M. and Chaniotakis N. A., Anal. Chim. Acta., 478, 77-84 (2003)).

SUMMARY OF THE INVENTION

The present invention is a fluoride-sensing cell that includes amembrane electrode made with a single-crystal pellet of trifluoridelanthanide series rare earth metals such as lanthanum, cerium,praseodymium, neodymium, promethium, samarium, or europium, doped withalkaline earth metals ion such as strontium, barium and calcium.Specifically, the membrane compositions are characterized by the formula(MF₂)_(1-x)(LnF₃), where (MF₂)_(1-x) is the proportion of alkaline earthand (LnF₃)_(x) is the proportion of the lanthanide-series metal. Theperformance of a strontium-doped lanthanum trifluoride single crystal issuperior to electrodes constructed of particles of the ingredients,whether by sintering or by incorporation into a polymeric matrix, forthe analysis of fluoride by an ion-selective electrode. The performanceof a strontium-doped lanthanum trifluoride single crystal is alsosuperior to one doped with europium: it has a lower detection limit anda wider pH range. Compared to the single crystal of trifluoridelanthanide doped with europium, the detection limit can be extended 5 to10-fold lower, to the 0.003 ppm fluoride range, and the pH range can beextended to pH 8 from pH 5.5 with 0.01 ppm fluoride detectable.

For fluoride measurement by fluoride ion-selective electrodes, manysample pre-treatment solutions additionally employ a masking agent topreferentially complex any potentially interfering species, e.g. di- andtrivalent cations, especially aluminum and iron, and thus remove themfrom the sample solution.Trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid (CDTA) has foundwide usage (Nicholson, K. and Duff, E. J., Anal. Lett. 14(A12), 887-912(1981)); and citric acid, disodium 1,2-dihydroxybenzene-3,5-disulphonate(Tiron), sodium ethylenediaminotetraacetate (EDTA) and, potassiumtartrate also have been described in use as complexing agents(Nicholson, K., Duff, E. J., Anal. Lett., 14 (A12), 887-912 (1981);David E. Davey, Dennis E. Mulcahy, Trevor J. Muggleton and Gregory R.O'Connell, Anal. Lett, 25 (3), 607-624 (1992); Akio, Yuchi, KazuhiroUeda, Hiroko Wada, Genkichi Nakagawa, Anal. Chim. Acta., 186, 313-318(1986). These complexing agents are used in the pH range of 5 to 5.5;however, the complexing ability may be increased by increasing the pH ofsample pretreatment solution above pH 5.5.

Instead of using acetate buffers in the range of pH 5 to 5.5, thisinvention uses a sample pretreatment solution providing a pH range of 5to 8. For example, one may use organic acids such as3-(N-morpholino)propanesulfonic acid (MOPS),3-(N-morpholino)-2-hydroxypropanesulfonic acid (MOPSO),N-(2-hydroxyethyl)piperazine-N′-(2-ethanesulfonic acid) (HEPES),2-(N-morpholino)ethanesulfonic acid (MES),piperazine-N,N′-bis(2-ethanesulfonic acid)(PIPES),3-[N,N-bis(2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid (DIPSO)and other biological buffers, also called Good's buffers (Good et al.Biochemistry, 5, 467-77(1966); Ferguson et al., Analytical Biochemistry104, 300-310 (1980)). Compared to acetate buffers, these buffers showimproved detection limits for fluoride measurement. Also in these buffersystems, complexing agents show stronger complexing properties,resulting in improved selectivity of fluoride measurement in presence ofinterference from aluminum and iron, for example. It should beunderstood that the invention is not limited to these buffers. Otherorganic or inorganic buffers may also be used. The invention alsocontemplates the use of a single compound, such as 5-sulfosalicylic acidas both a buffer and a complexing agent.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention description below refers to the accompanying drawings, ofwhich:

FIG. 1 is a cross section of an electrode embodying the principles ofthe invention;

FIG. 2 is a schematic view of a cell incorporating the electrode of FIG.1; and

FIGS. 3-8 are plots of the response of the electrode of the presentinvention, including comparisons with a prior electrode (Curve 1) andthe prior art (Curve 2).

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

Referring now to the drawing, there is shown in FIG. 1 and FIG. 2 anelectrode 20 embodying the principles of the present invention andcomprising an elongated, hollow tubular container or stem 22 open atboth ends. The stem typically is formed of a liquid-impervious,substantially rigid, electrically-insulating material, such asunplasticized polyvinylchloride, polytetrafluoroethylene, or the like,substantially chemically inert to salt solutions containing fluorideions with which the stem might be placed in contact.

One end of stem 22 is capped or sealed with a barrier disc or membrane24 formed of a substantially imporous, high-purity, crystallinefluoride. The membrane can be quite thick, for example, 0.25 inchalthough thinner structures are preferred. Membrane 24 can be sealedacross the one end of the stem 22 with an appropriate sealing compoundsuch as an epoxy or polyester resin. Alternatively, as shown, themembrane is mounted by an O-ring 26 disposed about the periphery of theopening in the stem, and held in pressed-fit against the O-ring byannular flange 27 of collar 28 threadedly mounted on the stem. Whencollar 28 is rotated in the proper direction, it advances axially,forcing membranes 24 in a tight fit against the O-ring, thus sealing theone end of stem 22. Both the O-ring and collar 28 are preferably made ofplastic material such as polyvinyl chloride.

Disposed internally of stem 22 and in electrical and physical contactwith the inner surface of membrane 24 is a charge transfer meansproviding a fixed concentration of ions. This means is shown as areference electrolyte 30, for example, an aqueous solution of KC1,saturated with AgC1, and 1 mmolar in fluoride from KF. Immersed inelectrolyte 30 is internal reference electrode 32, for example thewell-known Ag—AgC1 element. This combination of electrolyte 30 andreference electrode 32 provides for electrical contact with the internalinterface (i.e. the surface of the membrane contacting the referenceelectrolyte) at a substantially stable or fixed potential. The other,open, end of stem 22 is fitted with an annular cap 34 having an aperturein which is sealed the usual coaxial cable 36, the central conductor ofwhich is connected to internal reference electrode 32 and the peripheralconductor of which is intended to provide electrostatic shielding. Theouter surface of membrane 24 is exposed to the sample solution whosefluoride content is to be measured.

The present inventors have discovered that a membrane fashioned from asingle-crystal pellet of a lanthanide series trifluoride, doped withstrontium, provides an improved electrode response to fluoride ions,with a detection limit that can be extended to 10-fold lower, to the0.003 ppm, than a single-crystal pellet of trifluoride lanthanide dopedwith europium. FIG. 3 compares the response curves of a single-crystalpellet of lanthanide trifluoride doped with strontium and a prior artsingle crystal doped with europium.

Response time is also an important criterion for electrode performance.Although the membrane comprising a single crystal of lanthanumtrifluoride doped with europium has found wide usage for measurement offluoride ions, the electrode response is slow, especially in solutionsof less than 1 ppm fluoride, taking up to 5 minutes to stabilize. Thepresent membrane has a much shorter response time. FIG. 4 shows aresponse time comparison of a strontium-doped electrode and one dopedwith europium.

FIG. 5 shows the effect of pH on the response of fluoride electrodes insolutions with two different electrodes. The present electrode exhibitsa wider useable pH range for response to fluorides. It extends theuseable range up to pH 8, even in fluoride concentrations less then 1ppm. In the solution of 10⁻⁶ M fluoride ion (0.02 ppm), the electrodewith the present art made with strontium-doped crystals showed no effectof pH value change from 5 to 8, while the electrode of the prior artmade with Eu-doped crystals showed response deterioration with pHincrease from 5 to 8. In the solution of 10⁻⁵ M fluoride ion (0.2 ppm),the electrode with present art showed no effect from a pH value changefrom 5 to 9.5, while the electrode with old art showed responsedeterioration with pH increase from pH 5.5 to pH 9.

FIG. 6 shows the blank values of different buffers. The blank values,i.e., measurements in which the sample contains no fluoride ions and areindicative of the lower limit of detection of sample pre-treatmentsolutions for fluoride measurement. It can be seen that for the sameelectrode, buffers described above exhibit lower detection limits thanacetate buffers. Also, electrodes made of 5% strontium doped crystalsshow blank fluoride values less than 0.01 ppm in MES pH 5.4, MOPSO pH5.9, MOPS pH 6.3, or MOPS pH 7.2 buffers, while electrodes made of 0.5%Eu doped crystals show 0.01 ppm blank values only in pH 5.4 buffers.

With the pH range extended to pH 8, 5-sulfosalicylic acid (SSA), citricacid, tartaric acid, Tiron, EDTA and CDTA, for example, can be used asthe complexing agents. It is well known that complexing agents tend toimprove complexing power when pH values increase. (Anders Ringbom,Complexation in Analytical Chemistry, Interscience Publishers, 1963).Conditional constant of the complexing with metal ions is increased withthe increasing of pH values in solution. As an example, Table 1 liststhe conditional constants of complexing agents EDTA, CDTA and citratewith Al ion under different pH values. TABLE 1 Conditional constants ofsome complexing agents EDTA, CDTA and citrate with Al ion at differentpH values (from Anders Ringbom, Complexation in Analytical Chemistry,Interscience Publishers, 1963, p352) pH EDTA CDTA Citrate 2 1.8 0.2 34.1 2.8 1.8 4 6.2 5.5 5.2 5 8.2 7.6 8.6 6 10.3 9.4 11.3 7 12.5 10.8 13.68 14.5 12.3 15.6 9 16.5 14.3 17.6

FIG. 7 shows 0.1 ppm fluoride measurement in the presence of Al ioninterference with 0.1M SSA as a complexing agent at pH 5.4 and pH 7.1.With increasing pH from 5.4 (curve 1, acetate buffer) to 7.1 (curve 2,MOPS buffer), curve 2 (at pH 7.1) showed much less Al ion interferencefor fluoride measurement compared to curve 1 (at pH 5.4). Theselectivity of the present invention is improved greatly compared to theprior art of acetate buffers with complexing agenttrans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid (CDTA).

FIG. 8 shows a test error of 1 ppm fluoride in the presence of differentconcentrations of Al interference. It can be seen that selectivity withAl interference can be improved at least 10 fold compared to standardmethod (Method 4500F) based on the prior art. With the prior art, 2 ppmAl can cause 10% measurement error for 1 ppm fluoride, while with theinventive buffers, 2 ppm Al causes negligible error for 1 ppm fluoridemeasurement. If a 10% error is permissible, the present invention cantolerate 30 ppm Al in the solution, while the prior art can tolerantonly 2 ppm Al. Similar improvement has also been observed for otherinterference such as iron (III).

EXAMPLE 1

In example 1, the electrode was made of a single crystal of trifluoridelanthanum (LaF₃) doped with 5% m/m strontium (Sr). The single crystalwas then cut into a disc, about 8 mm in diameter and 1.6 mm thick. Thefinish on all surfaces was ground by a 320-mesh diamond abrasive. Thepellet was mounted over the end of a polystyrene tube and glued with anepoxy as permanent seal. The latter was filled with an aqueous solutionwith 1 molar fluoride in KC1 as well as saturated with AgC1. An Ag—AgC1electrode was placed in the internal solution. The single crystalsurface was polished to a mirror surface after epoxy was cured fully.

This electrode was tested in a configuration using an Ag—AgC1 externalreference electrode. A number of aqueous solutions of sodium fluoride atdifferent concentrations were tested. The response in mV for differentfluoride concentrations in sample solutions for the electrode and aprior art electrode are listed in Table 2. TABLE 2 Fluoride Response ofSingle Crystal trifluoride lanthanum doped with strontium vs. europiumin 1:1 acetate buffer solutions Fluoride ion Reading in mV, (LaF₃Reading in mV, (LaF₃ concentration, Single Crystal Single Crystal mol/LDoped with Sr) Doped with Eu) 10⁻¹ −100.1 −118.1 10⁻² −43.7 −61.6 10⁻³14.3 −3.4 10⁻⁴ 72.3 55.1 10⁻⁵ 132.1 116.9 10⁻⁶ 189.6 168 10⁻⁷ 228.3181.8

EXAMPLE 2

In example 2, the electrode of Example 1 was tested with a samplepre-treatment solution buffer, which has the following composition:

0.5 moles/liter MOPS where 0.25 moles/liters sodium form and 0.25moles/liter acid form,

1.0 moles/liter sodium chloride

0.1 moles/liter 5-sulfosalicylic acid or citric acid or tartaric acid

This solution has pH about 7. 0.5 moles/liter MOPS also can be made with0.5 moles/liter MOPS with acid form and then adjusted pH to 6.5 to 7.5with a sodium hydroxide solution, or MOPS with sodium, and then adjustedpH to 6.5 to 7.5 with addition of HCl acid to the solution.

EXAMPLE 3

In example 3, the electrode was tested in a sample pre-treatmentsolution, which has the following composition:0.2 moles/liter HEPESwhere 0.10 moles/liter sodium form and 0.10 moles/liter acid form;

1.0 moles/liter sodium chloride 0.1 moles/liter 5 sulfosalicylic acid orcitric acid or tartaric acid

This solution has pH about 7.5. 0.2 moles/liter HEPES also can be madewith 0.2 moles/liter HEPES with acid form and then adjusted pH to 7.0 to8.0 with addition of NaOH to the solution. It also can be made with 0.2miles/liter HEPES sodium form and then adjusted pH to 7.0 to 8.0 withaddition of HC1 into the solution.

EXAMPLE 4

In example 4, the electrode was tested in a sample pre-treatmentsolution, which had the following composition:

0.1 mole/liter MOPSO where 0.05 moles/liter sodium form and 0.05mole/liter acid form;

1.0 moles/liter sodium chloride

0.1 moles/liter 5-sulfosalicylic acid or citric acid or tartaric acid

This solution had a pH about 6.7. 0.1 moles/liter MOPSO also can be madewith 0.1 moles/liter MOPSO with acid form and then adjusted pH to 6.5 to7.0 with addition of NaOH to the solution. It also can be made with 0.1miles/liter MOPSO sodium form and then adjusted pH to 6.5 to 7.0 withaddition of HC1 into the solution.

EXAMPLE 5

In example 5, the electrode of example 1 tested in a samplepre-treatment solution, which had the following composition:

0.2 mole/liter MES where 0.1 moles/liter sodium form and 0.1 moles/literacid form;

1.0 moles/liter sodium chloride

0.1 moles/liter 5-sulfosalicylic acid or citric acid or tartaric acid

This solution has a pH about 5.5. 0.2 moles/liter MES also can be madewith 0.1 moles/liter MES with acid form and then adjusted pH to 5 to 6with addition of NaOH to the solution. It also can be made with 0.2miles/liter MES sodium form and then adjusted pH to 5 to 6.0 withaddition of HCl into the solution.

EXAMPLE 6

In example 6, the electrode is tested in a sample pre-treatmentsolution, which has the following composition:

0.5 mole/liter MOPS where 0.25 moles/liter sodium form and 0.25moles/liter acid form;

2.0 moles/liter sodium chloride

0.1 moles/liter 5-sulfosalicylic acid or citric acid or tartaric acid

This solution has a pH of about 7 with complexing agent 5-sulfosalicylicacid. Citric acid or tartaric acid, or other complexing agents also canbe used as a complexing agent. This sample pre-treatment solution can beused with up to 40 ppm Al and 200 ppm Fe (III).

1. A fluoride ion-selective electrode for monitoring the fluoride ionconcentration in a sample solution, said electrode comprising asingle-crystal membrane of a rare earth fluoride doped with alkalineearth ions.
 2. The electrode of claim 1 in which the rare earth fluorideis lanthanum fluoride and the alkaline earth ions are from the group ofstrontium, barium, and calcium ions.
 3. The electrode of claim 1 inwhich the dopant concentration is in the range of 0.1 to 20 percent m/m.4. A sample pre-treatment solution for use with the electrode of claim 1containing: A. a buffering agent capable of maintaining a pH in therange of 5-8; and B. a complexing agent capable of complexing aluminumand iron
 5. The sample pre-treatment solution of claim 4 in which thecomplexing agent and the buffering agent are the same compound.
 6. Thesample pre-treatment solution of claim 4 in which the buffering agentand complexing agent are different compounds.
 7. A sample pre-treatmentsolution as in claim 4 in which the buffering agent is an acid compoundselected from the group of MOPS, MOPSO, HEPES, MES, PIPES, BIS-TRIS andDIPSO.
 8. The sample pre-treatment solution of claim 4 in which thecomplexing agent is from the group of 5-sulfosalicylic acid, citricacid, tartaric acid, CDTA, EDTA, and Tiron.
 9. A fluoride ion-selectiveelectrode for monitoring the fluoride ion concentration in a samplesolution, said electrode comprising a single crystal membrane of a rareearth fluoride doped with alkaline earth ions, said electrode used witha sample pre-treatment solution containing a buffering agent capable ofmaintaining a pH in the range of 5-8 and a complexing agent capable ofcomplexing aluminum and iron.
 10. An electrode as in claim 9 in whichthe rare earth fluoride is lanthanum fluoride and the alkaline earthdopant is from the group of strontium, barium, and calcium ions.
 11. Theelectrode as in claim 9 in which the dopant ion concentration is in therange of 0.1 to 20 percent m/m.
 12. The sample pre-treatment solution asin claim 9 wherein the complexing agent and the buffering agent are thesame compound.
 13. The sample pre-treatment solution of claim 9 in whichthe complexing agent and buffering agent are different compounds.
 14. Asample pre-treatment solution as in claim 9 in which the buffer solutionis an compound selected from the group of MOPS, MOPSO, HEPES, MES,PIPES, BIS-TRIS and DIPSO.
 15. A sample pre-treatment solution as inclaim 9 wherein the complexing agent is selected from the group of5-sulfosalicylic acid, citric acid, tartaric acid, CDTA, EDTA and Tiron.